1. Field of the Invention
The present invention relates to a device and system for phase conjugate conversion and wavelength conversion.
2. Description of the Related Art
Owing to recent developments of low-loss silica optical fibers, various optical fiber communication systems each using such an optical fiber as a transmission line have been put to practical use. The optical fiber itself has a very wide band. However, a transmission capacity by the optical fiber is actually limited by a system design. The most important limitation is due to waveform distortion by chromatic dispersion occurring in the optical fiber. Further, the optical fiber attenuates an optical signal at a rate of about 0.2 dB/km, for example. Loss of the optical signal due to this attenuation has been compensated for by adopting an optical amplifier such as an erbium doped fiber amplifier (EDFA) that is a typical example.
The chromatic dispersion that is often referred to simply as dispersion is a phenomenon such that the group velocity of an optical signal in an optical fiber changes as a function of the wavelength (frequency) of the optical signal. In a standard single-mode fiber, for example, an optical signal having a longer wavelength propagates faster than an optical signal having a shorter wavelength in a wavelength region shorter than 1.3 μm, and the resultant dispersion is usually referred to as normal dispersion. In contrast, an optical signal having a shorter wavelength propagates faster than an optical signal having a longer wavelength in a wavelength region longer than 1.3 μm, and the resultant dispersion is usually referred to as anomalous dispersion.
In recent years, the nonlinearities of an optical fiber have received attention in association with an increase in optical signal power due to the use of an EDFA. The most serious nonlinearity that limits a transmission capacity is an optical Kerr effect occurring in an optical fiber. The optical Kerr effect is a phenomenon such that the refractive index of an optical fiber changes with the intensity of an optical signal. A change in the refractive index modulates the phase of an optical signal propagating in an optical fiber, resulting in the occurrence of frequency chirping which changes a signal spectrum. This phenomenon is known as self-phase modulation (SPM). Spectral broadening due to SPM occurs to cause further enlargement of the waveform distortion due to chromatic dispersion.
In this manner, the chromatic dispersion and the optical Kerr effect impart waveform distortion to an optical signal with an increase in transmission distance. Accordingly, to allow long-haul transmission by an optical fiber, the chromatic dispersion and the nonlinearity must be controlled, compensated, or suppressed.
As a technique for controlling the chromatic dispersion and the nonlinearity, the use of a regenerative repeater including an electronic circuit for a main signal is known. For example, a plurality of regenerative repeaters are arranged along a transmission line. Each regenerative repeater performs opto/electric conversion, regeneration, and electro/optic conversion in this order before the waveform distortion of an optical signal becomes excessive. However, this method has a problem that the regenerative repeater required is expensive and complicated, and that the electronic circuit included in the regenerative repeater limits the bit rate of a main signal.
As a technique for compensating for the chromatic dispersion and the nonlinearity, optical soliton is known. An optical signal pulse having an amplitude, pulse width, and peak power each accurately specified with respect to a given anomalous dispersion is generated, thereby balancing pulse compression due to both SPM induced by the optical Kerr effect and the anomalous dispersion and pulse broadening due to dispersion, so that an optical soliton propagates as maintaining its waveform.
As another technique for compensating for the chromatic dispersion and the nonlinearity, the application of optical phase conjugation is known. For example, a method for compensating for the chromatic dispersion of a transmission line has been proposed by Yariv et al. (A. Yariv, O. Fekete, and D. M. Pepper, “Compensation for channel dispersion by nonlinear optical phase conjugation” Opt. Lett., vol. 4, pp. 52-54, 1979). An optical signal is converted into phase conjugate light at the midpoint of a transmission line, and the waveform distortion due to chromatic dispersion in the front half of the transmission line is compensated by the waveform distortion due to chromatic dispersion in the rear half of the transmission line.
In particular, if the causes of phase fluctuations of electric fields at two points are identical with each other, and an environmental change inducing these causes is gentle during a light propagation time between the two points, the phase fluctuations can be compensated by locating a phase conjugator (phase conjugate light generator) at the midpoint between the two points (S. Watanabe, “Compensation of phase fluctuation in a transmission line by optical conjugation” Opt. Lett., vol. 17, pp. 1355-1357, 1992). Accordingly, the waveform distortion due to SPM can also be compensated by adopting the phase conjugator. However, in the case that the optical power distributions on the upstream and downstream sides of the phase conjugator are asymmetrical with respect thereto, the compensation for nonlinearity becomes incomplete.
The present inventor has proposed a technique for overcoming the incompleteness of the compensation due to the asymmetry of optical powers in the case of using a phase conjugator (S. Watanabe and M. Shirasaki, “Exact compensation for both chromatic dispersion and Kerr effect in a transmission fiber using optical phase conjugation” J. Lightwave Technol., vol. 14, pp. 243-248, 1996). The phase conjugator is located in the vicinity of a point on a transmission line such that a total dispersion or total nonlinear effect in a portion of the transmission line upstream of this point is equal to that in a portion of the transmission line downstream of this point, and various parameters are set in each minute section of the upstream and downstream portions.
By using a third-order nonlinear optical medium such as an optical fiber and a semiconductor optical amplifier, phase conjugate light can be generated by nondegenerate four-wave mixing. When signal light having an angular frequency ωS and pump light having an angular frequency ωP (ωP≠ωS) are supplied to the nonlinear optical medium, phase conjugate light (converted signal light) having an angular frequency 2ωP−ωS is generated by four-wave mixing based on the signal light and the pump light in the nonlinear optical medium, and this phase conjugate light is output together with the signal light and the pump light from the nonlinear optical medium.
The above term of “nondegenerate” used herein means that the wavelength of the signal light and the wavelength of the pump light are different from each other. Since the wavelength of the signal light, the wavelength of the pump light, and the wavelength (angular frequency) of the phase conjugate light satisfy the above-mentioned relation, wavelength conversion is performed simultaneously with the generation of the phase conjugate light.
The efficiency of conversion from the signal light to the phase conjugate light depends on the consistency of the polarization planes of the signal light and the pump light. However, since a general optical fiber transmission line has no capability of maintaining a polarization plane, the polarization state of the signal light to be converted varies with time. Accordingly, it is required that a device for phase conjugate conversion and wavelength conversion has no polarization dependence. The wording of “no polarization dependence” used herein means that the conversion efficiency is substantially constant irrespective of the polarization state of the signal light to be converted.
In the case that the device for phase conjugate conversion and wavelength conversion is applied to WDM (wavelength division multiplexing), a sufficiently broad conversion band is required to increase the number of channels that can be subjected to simultaneous conversion. The conversion band is defined as a maximum detuning wavelength (or detuning frequency) between pump light and signal light under the condition that phase conjugate light having a certain power can be obtained.